Multicolor display devices

ABSTRACT

The present invention concerns a multicolor display device, comprising a transparent substrate, fluorescent dye deposited in a dye layer onto the substrate by ink jet printing, and a source of radiation for illuminating said fluorescent dye. The present invention also concerns methods for creating such a device.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.F33615-94-1-1414 awarded by DARPA. The government has certain rights inthis invention.

FIELD OF THE INVENTION

This invention relates to display devices, and more particularly tomulticolor display devices having fluorescent dyes deposited by ink jetprinting.

BACKGROUND OF THE INVENTION

Display devices utilizing fluorescent media capable of absorbing lightand emitting it at a longer wavelength are known. See, e.g., U.S. Pat.No. 5,294,870 to Tang et al., entitled "Organic ElectroluminescentMulticolor Image Display Device," the entire disclosure of which ishereby incorporated by reference. However, the fluorescent media in suchdevices are deposited using patterning techniques such asphotolithography which are costly to perform. Hence, there exists a needfor a method by which fluorescent media can be inexpensively andaccurately deposited on a substrate to produce a multicolor displaydevice.

SUMMARY OF THE INVENTION

The present invention is directed to display devices, each comprising asubstrate and fluorescent dye deposited onto the substrate. The presentinvention further includes methods of making such display devicesutilizing ink jet printing techniques. Such devices are advantageous inthat the fluorescent dye can be deposited cheaply, over very large orvery small areas, with a high degree of resolution. Consequently, theycan be used in practically any application where display devices arepresently used.

In one embodiment of the present invention, red, green and/or bluefluorescent dyes are ink jet printed onto a transparent substrate tocreate an image with a predetermined configuration. The thus-createdimage is thereafter exposed to ultraviolet or other short wavelengthradiation to activate the dyes and create a luminous color display. Sucha display is "passive" in the sense that the image is fixed in theas-printed configuration.

In another embodiment of the present invention, red, green and/or bluefluorescent dyes are again ink jet printed in a predeterminedconfiguration onto a transparent substrate. A layer of transparent,conductive material is then deposited over the dyes. A layer of organic,blue light emitting device (OBLED) is thereafter deposited over thelayer of transparent conductive material, and a conductive layer isdeposited over the OBLED layer. Application of a potential across theconductive layers illuminates the OBLED layer, producing a blueemission, which stimulates fluorescent emission in the dyes, so long asthe energy of the blue emission is greater than the emission of thedyes.

In another embodiment of the present invention, red, green and bluelight emitting regions are ink jet printed in a predeterminedconfiguration onto a transparent substrate. The red and green lightemitting regions are formed by ink jet printing red and greenfluorescent dyes onto the substrate. No ink is deposited in the bluelight emitting regions. Rather, the blue light emitting regions are leftas empty spaces while the red and green fluorescent dyes are printed.After the red and green dyes are printed, a layer of transparent,conductive material is deposited over the red and green dyes and theempty spaces left for the blue light emitting regions. A layer oforganic, blue light emitting device (OBLED) is thereafter deposited ontothe layer of transparent, conductive material. Electrical contacts arethen placed on the OBLED in each of the red, green and blue lightemitting regions, to facilitate the application of voltage across theOBLED. The OBLED produces a blue emission in the blue light emittingregions, and further, stimulates fluorescent emission in the red andgreen light emitting regions thereby creating a luminous color display.In this embodiment, the red and green dyes preferably have strongabsorption in the blue, and preferably further have high blue-to-red andblue-to-green conversion efficiencies.

In a further embodiment of the present invention, red, green and bluelight emitting regions are ink jet printed in the form of pixels onto atransparent substrate. Each pixel has one of each of a red, green andblue light emitting region. This fourth embodiment is made in a mannersimilar to the third embodiment, the primary difference being that thelight emitting regions of the fourth embodiment are arranged intri-color pixels whereas the light emitting regions of the thirdembodiment are arranged in some predetermined configuration.

The displays of the present invention can be used in a wide variety ofproducts including a computer, a television, a telecommunications devicewhich incorporates a screen such as a telephone, a vehicle, a billboardor sign, or a large area wall, theater or stadium screen. Moreover,because the display devices of the present invention can be made usingany sufficiently flat substrate, it is contemplated the devices can beused in xerography, thereby eliminating the need for lensing in printerbars.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multicolor display device,according to a first embodiment of the invention.

FIG. 2 is a cross-sectional view of a multicolor display device,according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the construction of highresolution, full-color displays using printing technologies that deliversmall amounts of liquid inks to a substrate in a specified pattern(referred to herein as "ink jet printing").

The liquid inks of the present invention preferably comprise selectedfluorescent dyes and a host matrix in a liquid carrier medium. Theliquid carrier medium is preferably water, an alcohol such as methanol,ethanol, or isopropanol, or mixtures of the same. The particular carriermedium is typically selected based on its ability to molecularlydisperse the fluorescent dyes and on its compatibility with theparticular materials with which it comes into contact.

The amount of host matrix is ordinarily selected to yield a viscositycompatible with the ink jet printing process and preferably ranges fromabout 2 to about 7 wt %. The amount of dye is selected such that it ispresent in an amount sufficient to give good color intensity, while notbeing so high that the dye molecules begin to aggregate reducingluminescence intensity. Preferred amounts of dye range from about 0.1 toabout 6 wt % of the matrix.

In general, liquid inks of the present invention contain one or moredyes that fluoresce in red, green or blue, which are used to generate aluminescent region of the appropriate hue. The color of the luminescentregion is dictated by the fluorescence energy of the dye and therelative proportions of the same. The dyes are preferably chosen tooptimize the saturation (i.e., narrow lines at about 460, 520 and 650 nmfor blue, green and red, respectively), giving a wide color range forthe devices to be manufactured.

Preferred dyes with predominantly blue emission include:8-anilino-1-napthalenesulfonic acid, 1,3-diphenyl-1,3-butadiene,diphenylhexatriene, Hoescht 33258, Hoescht 33324, thioflavin T,diamidino-2-phenylindole*2HCL, coumarin 152, coumarin 20, coumarin 2,coumarin 339, coumarin 1, coumarin 138, coumarin 102, coumarin 314, andcoumarin 30.

Preferred dyes with predominantly green emission include: acridineorange, acridine yellow, acriflavin, dichlorofluorescene,3,6-diaminoacridine, fluoresceneisothiocyanate, lucifer yellow,quinacrinerhodamine 123, quinacridone, dimethylquiancridone,fluorescene, rhodamine 110, rhodamine 6G, and coumarin 6.

Preferred dyes with predominantly red emission include: xylenol orange,lumogen red, cresyl violet, diethylthiacarbocyanine, ethidium bromide,oxazine 170, nile blue, oxazine 1,1,3-bis[4-(dimethylamino)phenyl]-2,4-dihydroxycyclobutenediylium, and1,3-bis[4-(dimethylamino)-2-hydroxyphenyl]-2,4-dihydroxycyclobutenediylium.

In one embodiment, the liquid inks of the present invention are placedinto the wells of an ink jet printer, and mixed at individual points onthe substrate in ratios appropriate to yield the colors of the desiredimage at the resolution of the printer (commonly 300-600 dpi).Alternatively, green, blue and red groupings are deposited side-by-sideas individual pixels of a full color display. The thickness of thedeposited dye layer will be adjusted to optimize luminescent intensity.

The presently preferred ink jet printing device is an Epson Stylus Color500. While available in a printer, a preferred embodiment of theinvention involves the use of a stylus within a plotter. Although moreexpensive, a plotter improves operation by increasing precision,reducing indexing difficulties (e.g., in the event multiple layers aredeposited) and providing nonlinear features (such as interconnects).

The host matrix for the fluorescent dyes of the invention can compriseeither polymeric materials or small molecules that readily form stableglassy thin films. Examples of polymeric matrix materials include:polymethylmethacrylate, polyvinylcarbazole, polybutadiene, andpolyesters. An example of a small molecule isN,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine. Thisis a good glass forming material that can be used as a hole transporterin OLEDs and has its maximum absorption in the UV part of the spectrum.The choice of matrix materials will depend, among other things, on thestability of the different materials under printing conditions, theirability to transmit the ultraviolet light that is used to fluoresce thedyes as well as the light produced by the dyes, and their ability toresist phase separation and stabilize the dyes with respect toaggregation.

The wavelength of the radiation used to fluoresce the dyes is preferablymaximized to reduce its energy (and thus its tendency to degrade thedisplay), but must be of greater energy than the light produced by thedyes. For this reason, the wavelength of the radiation is usually in theblue to near-UV range.

The substrate should be dimensioned such that it is flexible enough toaccommodate its use in an ink jet printing device and should efficientlytransmit ultraviolet radiation, visible radiation or both, depending onthe application. The substrate preferably transmits visible and nearultraviolet radiation, while filtering out higher energy ultravioletradiation. Preferred materials are flexible polyester and glass films(such as Pyrex™), with glass being more preferred due to its low oxygenpermeability.

A first embodiment of the present invention is shown in FIG. 1. In thisembodiment, a luminous color display (10) is made by ink jet printingregions of fluorescent dye (11) onto the front surface of a substrate(12). The front surface of substrate (12) is thereafter exposed to blueor UV radiation, thus stimulating fluorescent emission of dye (11). Thecolor of the dye region upon exposure can be controlled by varying therelative amounts of red, green and blue dye in the region. Substrate(12) should be transparent to the colors produced by the excited dye(11), while preferably filtering out any high energy UV radiation.

The dye can also be illuminated from the rear surface, with thesubstrate separating the dye from the radiation source. In this case,the substrate should be transparent to the wavelengths of the blueand/or ultraviolet light that excite the dye, while preferably filteringout high energy UV radiation.

Alternatively, an additional layer can be provided, sandwiching the dye.In this case, one layer separates the dye from the viewer and the otherseparates the dye from the illumination source. The materials describedabove are preferred for both layers. The dye can be deposited on eitherof these layers.

The source of the blue/ultraviolet radiation is not critical. Forexample, it can be a conventional source such as a fluorescent tube(e.g., a "black light" ). As another example, the source can be a simpledevice such as a planar OBLED layer sandwiched between two planarconductor layers (one of which is transparent to the illuminationradiation). A more elaborate version of such a device (e.g., onecontaining individual electrical connections for each dye region) isdiscussed in detail below. The ultimate wavelength of the radiationemitted by the source should be shorter than the shortest wavelength oflight emitted by the dye.

The display of FIG. 1 is "passive" in that it is limited to the printedconfiguration, and individual printed regions of fluorescent dye are notindividually illuminated. One advantage of this device is that noindividual electrical connections are needed for each region.

Additional embodiments of the device structure of the present inventionare constructed based on the arrangements shown in FIG. 2. In theseembodiments, red and green fluorescent dyes (21) are ink jet printedonto a transparent substrate (22) such as glass; a transparent,conductive layer (23) is deposited over the red and green dye; anorganic blue light emitting device (OBLED) layer (24) is deposited overthe transparent, conductive layer; and electrical contacts (25) aredeposited onto the OBLED layer.

A preferred material for the transparent, conductive layer (23) isindium-tin oxide (ITO). One desirable property indium-tin oxide is itsability to filter out destructive, high energy ultraviolet radiation,while being transparent to visible and near-ultraviolet radiation.

Layer (23) can be formed by means of conventional sputtering or electronbeam vapor deposition methods, and typically ranges in thickness fromabout 1000 to about 4000 Å. Below a certain thickness the resistance ofthe layer will begin to suffer, while above a certain thickness marginalutility becomes negligible. The deposition of layer (23) is preferablyconducted under vacuum.

After conductive layer (23) is deposited, OBLED layer (24) is preferablydeposited by thermal evaporation methods to a thickness which is often400-1000 Å. The ultimate thickness will depend upon the OBLED.Preferably, this thickness will be as thin as possible to lower thevoltage of the device, without significantly compromising quantumefficiency. The deposition of layer (24) is preferably conducted undervacuum. It is preferred that the device not be exposed to moisture,oxygen or other contaminants between the deposition of layers (23) and(24).

OBLED layer (24) is made from any suitable blue light-emissive organiccompounds such as, for example, metal bidentate ligand complexes, andaromatic and heterocyclic polymers, as hereinafter described.

As seen in PCT International Publication Number WO 96/19792 entitled"Multicolor Organic Light Emitting Devices," the disclosure of which ishereby incorporated by reference, the metal bidentate complexes whichmay be used for layer (24) have the formula MDL⁴ ₂ wherein M is selectedfrom trivalent metals of Groups 3-13 of the Periodic Table andLanthanides. The preferred metal ions are Al⁺³, Ga⁺³, In⁺³ and Sc⁺³. Dis a bidentate ligand such as 2-picolylketones, 2-quinaldylkentones and2-(o-phenoxy) pyridine ketones. The preferred groups for L⁴ includeacetylacetonate, compounds of the formula OR³ R wherein R³ is selectedfrom Si and C, and R is selected from hydrogen, substituted andunsubstituted alkyl, aryl and heterocyclic groups; 3,5-di(t-bu) phenol;2,6-di(t-bu) phenol; 2,6-di(t-bu) cresol; and H₂ BpZ₂. By way ofexample, the wavelength resulting from measurement of photoluminescencein the solid state of aluminum (picolymethylketone) bis [2,6-di(t-bu)phenoxide] is 420 nm. The cresol derivative of this compound alsomeasured 420 nm. Aluminum (picolylmethylketone) bis (OsiPh₃) andscandium (4-methoxy-picolylmethylketone) bis (acetylacetonate) eachmeasured 433 nm, while aluminum [2-(O-phenoxy)pyridine] bis[2,6-di(t-bu) phenoxide] measured 450 nm.

Polymers of aromatic and heterocyclic compounds which are fluorescent inthe solid state may be used for layer (24). Examples of such polymersinclude poly(phenylene), and poly(N-vinylcarbazole).

Additional OLED materials are known in the art (see, e.g., U.S. Pat. No.5,294,870 to Tang et al., entitled "Organic ElectroluminescentMulticolor Image Display Device"; Hosokawa et al., "Highly efficientblue electroluminescence from a distyrylarylene emitting layer with anew dopant," Appl Phys. Lett., 67 (26) Dec. 25, 1995, pp. 3853-3855;Adachi et al., "Blue light-emitting organic electroluminescent devices,"Appl. Phys. Lett., 56 (9) Feb. 26, 1990, pp. 799-801; Burrows et al.,"Color-Tunable Organic Light Emitting Devices," Apl Phys. Lett., Vol.69, Nov. 11, 1996, pp. 2959-2961). The entire disclosures of thesereferences are hereby incorporated by reference.

Distyrylarylene derivatives such as those described in Hosokawa et al.are a preferred class of compounds. Other preferred OLEDs are describedin the copending applications discussed below.

The deposition of electrical contacts (25) may be accomplished by vapordeposition or other suitable metal deposition techniques. A preferredmethod of depositing such contacts is by ink jet printing as disclosed,for example, in U.S. Pat. Nos. 4,668,533, 5,132,248 and 5,266,098, thedisclosures of which are hereby incorporated by reference in theirentireties. These electrical contacts may be made from indium, platinum,gold, silver or combinations such as Ti/Pt/Au, Cr/Au or Mg/Ag. Mg/Agcontacts are preferred.

The embodiments discussed above in connection with FIG. 2 havesubstantially the same device structure, the primary difference beingthat the light emitting regions of one embodiment are arranged inpixels, and is therefore useful for creating active displays such asflat-screen monitors and the like. In comparison, the light emittingregions of the other embodiment are arranged in some predeterminedconfiguration. This embodiment is therefore compatible with "writing"special purpose logos, alpha numerics, segmental displays using red,green and blue regions as described above, or mixing appropriate amountsof red, green and blue in a given region to achieve a special color forthat part of the display.

In view of the above, it can be seen that the display devices of thepresent invention are appropriate for an extremely wide variety ofapplications including billboards and signs, computer monitors,telecommunications devices such as telephones, televisions, large areawall screens, theater screens and stadium screens.

The subject invention as disclosed herein may be used in conjunctionwith the subject matter of co-pending applications "High Reliability,High Efficiency, Integratable Organic Light Emitting Devices and Methodsof Producing Same", Attorney Docket No. 10020/1; "Novel Materials forMulticolor LED's", Attorney Docket No. 10020/2; "Electron Transportingand Light Emitting Layers Based on Organic Free Radicals", AttorneyDocket No. 10020/3; "Red-Emitting Organic Light Emitting Devices(LED's)" Attorney Docket No. 10020/5; and "High Efficiency Organic LightEmitting Device Structures," Attorney Docket No. 10020/7; eachco-pending application being filed on even date herewith, and each beingherein incorporated by reference in its entirety. The subject inventionas disclosed herein may also be used in conjunction with the subjectmatter of co-pending applications U.S. Ser. Nos. 08/354,674; 08/613,207;08/632,316; 08/632,322; 08/693,359; 60/010,013; and 60/024,001; and eachis also herein incorporated by reference in its entirety.

The following Examples are merely illustrative of the present inventionand are in no way intended to limit the scope of the present invention.

EXAMPLE 1 Generating a Passive Picture to be Back Lit with UV.

In order to print a fluorescent image or pixel array with an ink jetprinter or other ink delivery system, the inks are first prepared tomatch the optimal viscosity and other solution properties of the chosenprinter. These inks consist of a carrier solvent, roughly 1-10 weight %matrix material and 0.001-0.05 weight % fluorescent dye. The dye ischosen to achieve a desired hue (typically red, green or blue) and thematrix material is chosen to give a stable film which supports the dyesand prevents aggregation. The three wells of the printer are chargedwith the red, green and blue inks. The image is printed directly onto apolymeric or glass substrate. The inks are mixed by the ink jet printerat each pixel, if needed to achieve the appropriate color. Thebrightness at each pixel is adjusted by varying the total amount of inkdeposited at each pixel. A small amount of ink will allow most of theirradiating light to pass through the film giving very little visiblelight. A larger amount of ink deposited will give a significantabsorbance and thus a relatively larger amount of visible light from dyefluorescence. The color of each pixel is strictly determined by theratios of the individual red, green and blue inks.

EXAMPLE 2 Generating a Passive Picture Back Lit with OLEDs

To print a fluorescent image or pixel array with an ink jet printer orother ink delivery system, the inks are first prepared to match theoptimal viscosity and other solution properties of the chosen printer.These inks consist of a carrier solvent, roughly 1-10 weight % matrixmaterial and 0.001-0.05 weight % fluorescent dye. The dye is chosen toachieve a desired hue (typically red, green or blue) and the matrixmaterial is chosen to give a stable film which supports the dyes andprevents aggregation. The three wells of the printer are charged withthe red, green and blue inks. The image is printed directly onto apolymeric or glass substrate. The inks are mixed by the ink jet printerat each pixel, if needed to achieve the appropriate color. The color ofeach pixel is strictly determined by the ratios of the individual red,green and blue inks. The thickness of the fluorescent films will bechosen to achieve a transparency of less than 10% at the intendedirradiation wavelength. A layer of transparent conducting material isthen applied to the entire printed substrate. This conductive materialis indium-tin oxide (deposited by sputtering) or a conducting polymer(applied by spray on or other large area technique) or any othertransparent conductive material that may be used as the anode in anOLED. Multiple organic layers are then deposited over the entiresubstrate to make an active multilayer OLED structure. The compositionand structure of these organic multilayers are chosen to match theoutput of the OLEDs to the absorption spectra of the chosen red, greenand blue dyes, and are well known to those skilled in the art offabrication or organic light emitting devices. A mask is then appliedand a film of a low work function metal is deposited above each pixeldefined by the ink jet printer. Applying a bias between the conductivefilm and the metal electrode gives light which stimulates the dye regiongiving red, green and blue emission. The brightness at each pixel iscontrolled by setting the current level at the OLED. The color of agiven pixel is controlled by the ratio of red to green to bluefluorescent dyes in the film.

EXAMPLE 3 Generating a Pixel Array Back Lit with OBLEDs

To print a fluorescent image or pixel array with an ink jet printer orother ink delivery system, the inks are first prepared to match theoptimal viscosity and other solution properties of the chosen printer.These inks consist of a carrier solvent, roughly 1-10 weight % matrixmaterial and 0.001-0.05 weight % fluorescent dye. The dye is chosen toachieve a desired hue (red and green) and the matrix material is chosento give a stable film which supports the dyes and prevents aggregation.The wells of the printer are charged with the red and green inks. Theimage is printed directly onto a polymeric or glass substrate. Theprinter is used to deposit individual red and green fluorescent elementson the substrate. The thickness of the red and green fluorescent filmsare chosen to achieve a transparency of less than 10% at the intendedirradiation wavelength. A layer of transparent conducting material isthen applied to the entire printed substrate. This conductive materialcan be indium-tin oxide (deposited by sputtering) or a conductingpolymer (applied by spray on or other large area technique) or any othertransparent conductive material that may be used as the anode in anOLED. Multiple organic layers are then deposited over the entiresubstrate top make an active multilayer OBLED structure. The compositionand structure of these organic multilayers are chosen to match theoutput of the OBLEDs to the absorption spectra of the chosen red andgreen dyes, and are well known to those skilled in the art offabrication or organic light emitting devices. A mask is then appliedand a film of a low work function metal is deposited above each pixeldefined by the ink jet printer. Applying a bias between the conductivefilm and the metal electrode gives light which stimulates the dye regiongiving red or green emission, whose intensity is directly dependent onthe intensity of the OBLED used in irradiating it. The brightness ateach pixel is controlled by setting the current level at the OBLED. Theregions of the substrate that are not covered with the red or greenfluorescent dye will be open and blue light from the OBLED will betransmitted. Color mixing is achieved controlling the intensity of theindividual red, green and blue pixels.

We claim:
 1. A display, comprising:a transparent substrate; fluorescentdye-containing material deposited in a dye layer onto said substrate byink jet printing; and a source of radiation for illuminating saidfluorescent dye; wherein said display is passive.
 2. The display ofclaim 1, whereinsaid substrate has a front side and a rear side; saidfluorescent dye-containing material is provided on the front side of thesubstrate; and said source is positioned to illuminate the rear side ofthe substrate.
 3. The display of claim 1, wherein said fluorescentdye-containing material comprises one or more fluorescent dyes and amatrix material.
 4. The display of claim 3, wherein said dye is presentin an amount ranging from about 0.1 to about 6 wt % relative to saidmatrix material.
 5. The display of claim 3, wherein said matrix materialis selected from polymethylmethacrylate, polyvinylcarbazole,polybutadiene, polyesters andN,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine.
 6. Thedisplay of claim 1, wherein said substrate is selected from glass andpolyester.
 7. A computer incoporating the display of claim
 1. 8. Atelevision incoporating the display of claim
 1. 9. A large area wall,theater or stadium screen incoporating the display of claim
 1. 10. Abillboard or sign incoporating the display of claim
 1. 11. A vehicleincorporating the display of claim
 1. 12. A printer incoporating thedisplay of claim
 1. 13. A telecommunications device incorporating thedisplay of claim
 1. 14. A telephone incorporating the display ofclaim
 1. 15. The display of claim 1, wherein said radiation comprisesultraviolet radiation.
 16. The display of claim 15, wherein said sourceof radiation comprises a fluorescent tube.
 17. The display of claim 1,wherein said radiation comprises visible radiation.
 18. The display ofclaim 17, wherein said radiation comprises blue radiation.
 19. Thedisplay of claim 1, wherein said source of radiation comprises anorganic light emitting material.